where can Grandma take the kids in South Birmingham?

I have my 3 granddaughters 8, 5 and 18 months, one day a week each during the holidays for some lovely 1-2-1 time with Grumbo. I know all the local parks, Mac centre at Cannon Hill, Hickory Dickory's etc but wondered if there are any unusual activities suitable that don't cost the earth for this poor pensioner? I am mobile within limits as patrol is pretty dear now. I took the oldest one fruit-picking yesterday which she loved but it was miles out and ended up paying £20 for a few plums and raspberries! (I love Mums net, wish I had it when my son was small).